In a steam turbine generator system, the turbine is normally maintained at a constant speed and steam flow is varied to adjust the torque required to meet the electrical load imposed on the generator. The main steam control system is designed to accommodate normal changes in load demand and to smoothly adjust the turbine operating conditions to these changes. However, if the electrical load is suddenly lost or substantially reduced, requiring a commensurate reduction in steam flow to prevent turbine overspeed, the main control system usually does not have the ability to respond quickly enough to sharp variations in load demand, especially in high power to inertial ratio turbine systems.
As is well known, steam flow is directed into large turbines through multiple nozzle chambers, and steam admission into the nozzle chambers is regulated by valves which open to admit steam from steam supply conduits into the nozzle chambers, and close to obstruct the flow thereto. Variations in turbine design include full-arc admission units in which every first stage nozzle is active at all load conditions, and partial-arc units in which the number of nozzles is varied in response to load changes.
Full-arc admission turbines are usually designed to accept exact steam conditions at a rated load in order to maximize efficiency. Steam is admitted through all of the inlet nozzles, and the power output is controlled by throttling the main flow of steam from the generator to the turbine inlet. When power is decreased from the optimum level, however, there is an overall decline in efficiency system because throttling the flow of steam reduces the energy available for performing work.
More efficient control of turbine output than is achievable by the throttling method has been realized by the partial-arc admission system, wherein the inlet nozzles are separated into discrete groups and are contained in individual chambers. A relatively high efficiency is attainable by sequentially admitting steam through individual nozzle chambers with a minimum of throttling, rather than by throttling the entire arc of admission. (When multiple valves are used to regulate steam flow into a single nozzle chamber, typically these valves modulate together.)
Recent economic developments affecting the electric power industry have generated a significant need for cycling capacity. Cycling refers to either load-following operation, two-shifting (on-off) operation, or a combination of both. In particular, there is substantial interest in low-load overnight operation, typically in the range of 10 percent to 15 percent of capacity. This type of operation, however, presents certain operating problems, including an increase in the potential for low cycle thermal fatigue. Numerous published studies have demonstrated that this problem can be minimized, and the benefit of improved, or lower heat rate, at low load can be achieved, by the adoption of sliding throttle pressure operation. More particularly, a hybrid mode of sliding pressure operation has been recommended. In hybrid mode operation of a partial-arc admission turbine, the turbine is operated in the upper load ranges by activating the control valves at constant throttle pressure. As load is reduced, when a particular valve point is reached, valve position is held constant and throttle pressure is reduced to achieve further load reductions. (A valve point is defined as a state of steam admission in which each active valve is completely open or each inactive valve is completely closed.)
The lowest or optimum heat rates are achieved when the transition from constant to sliding throttle pressure operations occurs at the point where half the control valves are wide open and half are closed. For turbines in which the arc of admission is 100 percent at maximum load, the transition occurs at 50 percent admission. Setting the minimum admission point at 50 percent on turbines without individual valve actuators can achieve all the benefits of hybrid operation while in load-following mode.
The foregoing strategy, however, does not meet the requirements of two-shift, on-off operation. It has been found to be beneficial to operate the turbine at full-arc admission during start-up. Furthermore, tests have revealed that if a transition is made from full-arc to partial-arc admission during loading, in conjunction with sliding throttle pressure operation, there is an increase in rotor life as compared to full-arc admission operation all the way to full load. These tests demonstrate that valve transfer capability, from full-arc to partial-arc admission and vice versa, is extremely desirable for units employed in cyclic duty operation. This is best accomplished by the use of individual valve actuators.
Many older steam turbine units do not have individual control valve actuators, nor do their steam chests have sufficient space between the valves to accommodate individual actuators. One solution would be the wholesale but costly and time-consuming replacement of entire steam chests to permit installation of individual actuators. A less expensive method of transferring between a full-arc admission mode and a partial-arc admission mode for both internal and external bar lift steam chests has been presented in U.S. Ser. No. 217,515 filed 7/11/88, and issued Apr. 11, 1989 as U.S. Pat. No. 4,819,435, assigned to Westinghouse Electric Corporation.
There are in present use a substantial number of older steam turbine units that use a camshaft and cams to actuate the control valves, rather than internal or external bar lift means. These existing units do not have the desired valve transfer capability, from full to partial-arc admission, and vice versa, nor is there sufficient space within the steam chests to permit the installation of individual actuators. One method of transferring from full-arc admission at initial loading to partial-arc admission at some level of load uses a pilot valve or throttle valve bypass. During initial loading, the control valves are wide open and the pilot valve or throttle valve bypass controls the steam flow, thereby achieving full-arc admission. Because of serious erosion of these small pilot valves, some users have changed this procedure to one keeping the throttle valve wide open and increasing the minimum arc of admission to that corresponding to half the control valves being wide open. There is a heat rate (efficiency) penalty by doing this, because throttling of the control valves occurs at loads below the point at which the first half of the valves are wide open. In addition, 50 percent effective admission at start-up, even with sliding pressure, does not result in optimum rotor life. One proposal to improve heat rate utilizes a cam that opens two valves simultaneously in a given steam chest.
Accordingly, it is desirable to provide a method and apparatus for a valving sequence for these units, analogous to the above referenced U.S. Patent No. 4,819,435 which will make possible valve transfer from approaching full-arc admission to a partial-arc admission mode and vice versa, to render these turbines more suitable for cycling operations, without the expense of providing individual activation for each valve which would necessitate replacement of an entire steam chest. The existing art as to cam-driven control valves includes three alternative configurations. One type of turbine unit uses cam-driven valves having only a single steam chest located in the top cover of the outer turbine cylinder. Another type has steam chests in both base and top covers. The units with only top cover steam chests typically have six control valves with the outermost valves supplying nozzle chambers in the base half. Usually these two valves open together and are followed by the remaining four opening sequentially. Units with a steam chest in both top and base covers have a total of either six or eight valves, divided equally between each steam chest.